It's probably an issue between him and his acquirer. Visa has nothing to do with it.

The card will have a BIN number (usually first 4 numbers). That's a Bank Information Number, and defines which FI was the supplier. So Visa might be the payment brand, but the supplier could be RBC, or TD, or Chase bank, or MBNA...whatever.

International banks may have issues processing certain BIN numbers because they don't have agreements with the acquirer to remediate the payment with the supplier institution.

In IT terms you might think of it as a routing problem. His acquirer might not know how to map a BIN number back to a financial institution.